This invention relates to synthetic aperture radar (SAR) and more particularly to a multiple-look SAR for spacecraft applications which makes use of excess azimuth bandwidth in radar echo signals to produce multiple-look SAR images and provide real-time analysis of the antenna electric boresight.
The utility of spaceborne SAR has been extensively investigated in the past decades. Because of the capability of microwaves to penetrate through clouds and the unique contrast characteristics in SAR imagery, radar imaging is considered particularly useful for surface topographic mapping and for all-weather sea state observations. To extend the utility of spaceborne SAR so that the imaging radar could also be used as a global environmental monitoring device, a series of earth and planetary spacecraft may be launched with on-board imaging radars designed to be able to produce imagery with 25 meter resolution and 100 km swath width on the earth's surface. The high resolution and wide swath coverage imply an extremely high data acquisition rate. The large amount of SAR data acquired must be processed to produce images in a satisfactory format. Currently available SAR processors are not able to produce the imagery in a timely and economic manner. An effective way to meet the data processing needs in future anticipated operational radar imaging missions is to employ on-board SAR processors. Such on-board processors would produce SAR imagery from echo signals in real-time. Not only can the transmission bandwidth for imagery data be reduced by a large factor (compared with the unprocessed raw data transmission rate), but direct image transmission to users in the vicinity of the sensor also simplifies the data handling and distribution procedures.
A problem with designing a real-time spaceborne SAR processor is to provide multiple look images and real-time analysis of the antenna electric boresight to derive accurate processing parameters. It is thus desirable to produce time multiplexed single-look image lines from raw radar data storage in real time with a size of azimuth time-bandwidth product corresponding to that of a minimum resolution single-look image. Such SAR has not heretofore been available. Consequently, an object of this invention is to provide a real-time SAR processor which produces multiple-look images and which is capable of real-time analysis of the antenna electric boresight to derive accurate processing parameters.
The Doppler phase history in target impulse response for SAR is directly related to the radar antenna look angle that determines the target area under illumination. The process of tracking the Doppler center frequency and electronically compensating the frequency offset is commonly referred to as "clutterlock". Proper clutterlock is important to the satisfactory reduction of SAR data into imagery. This is particularly important to spacecraft applications which are expected to employ very large size antenna arrays--about 10 meters. At a typical frequency range, L to X bands, the along-track beamwidth is very narrow as compared to the uncertainty in the antenna attitude. To maintain an adequate image signal-to-noise ratio, it is therefore necessary to perform the clutterlock operation to refine the antenna attitude predicts. In a typical case in which clutterlock must be applied, the along-track beamwidth is approximately one degree while the attitude uncertainty in real-time predicts is about .+-.0.5 degree.